This invention relates to a chemical conversion treatment liquid for forming a chemical conversion film on the surface of a steel material such as a Cr-containing steel and to a surface treatment method employing the liquid.
The present invention also relates to a surface treated steel material which is manufactured using such a surface treatment method and particularly to a surface treated steel material having excellent seizing resistance.
Chemical conversion treatment is a type of treatment in which the surface of a steel material and a corrosive solution are chemically reacted with each other to form a film of a corrosion product having good adhesion to the surface of the steel material. Chemical conversion treatment is frequently referred to as phosphate treatment, chromate treatment, oxalate treatment, etc. depending upon the type of corrosive solution which is employed.
However, a chemical conversion film cannot be formed by such conventional chemical conversion treatment on a steel such as a high-Cr steel.
As disclosed in Japanese Published Unexamined Patent Application 57-82478, for example, a method is known in which chemical conversion treatment is performed on the surface of a steel material using a chemical conversion treatment liquid based on an alkali metal phosphate and containing a titanium compound and a chlorate salt, after which further chemical conversion treatment is performed using a chemical conversion treatment liquid containing a zinc phosphate. However, this method has the disadvantage that treatment must be performed two times. Moreover, this method cannot form a sound chemical conversion film of a phosphate on a high-Cr steel such as a 13% Cr steel.
Japanese Published Unexamined Patent Application 5-40034 discloses a method of surface treatment using a chemical conversion treatment liquid containing manganese and phosphoric acid to which fluoride ions are added. However, even with this method, it is not possible to form a chemical conversion film on a Cr-containing steel.
Oil well steel pipes are connected to each other through couplings. For this purpose, male threads formed on the ends of oil well steel pipes are mated with female threads formed on the inner surface of couplings, and the threads are tightened to form a gas- and liquid-tight joint connecting the pipes. At the time of tightening, a large torque is applied to the threads, so it becomes easy for defects such as galling to occur on the thread surface, which reduces the number of times which the oil well steel pipes can be repeatedly connected to each other. In addition, if corrosion occurs on the thread surface, it becomes difficult to guarantee an adequate gas tightness and liquid tightness.
Accordingly, in the past, the thread surface of a threaded joint for oil well steel pipes made of a Cr-containing steel was plated with a soft metal such as Cu in order to prevent galling. However, due to the man hours required for plating, the plating method is not satisfactory, and there is room for improvements.
Thus, there has been a demand for a technique which can stably form a sound chemical conversion film such as a zinc phosphate film or a manganese phosphate film on the surface of a Cr-containing steel.
An object of this invention is to provide a chemical conversion treatment liquid which can stably form a chemical conversion film of a phosphate even on the surface of a Cr-containing steel such as one containing 0.5-13% Cr.
Another object of this invention is to provide a method of manufacturing a surface treated steel material in which the surface treatment is performed in such a manner that a chemical conversion film of a phosphate can be stably formed even on the surface of the above-described Cr-containing steel.
Yet another object of this invention is to provide a surface treated steel material having such a chemical conversion film of a phosphate formed thereon.
The present inventors found that the addition of a potassium compound to a phosphate-type chemical conversion treatment liquid results in a significant improvement in the film-forming ability and makes it possible to stably form a phosphate-type chemical conversion film even on a Cr-containing steel, on which it has been difficult to form a chemical conversion film.
Based on these findings, the present inventors continued research and development and further found that such effect of a potassium compound and particularly of potassium tetraborate is generally seen in chemical conversion films formed by chromate treatment, oxalate treatment, and the like, and completed the present invention.
In a broad sense, the present invention is a surface treated steel material comprising a steel material and a chemical conversion film formed on at least a portion of the surface of the steel material, the chemical conversion film containing potassium in an amount of 0.1-1000 mg/m2 and having a thickness of 5-50 micrometers and preferably 5-35 micrometers.
In a preferred embodiment of the invention, the chemical conversion film is a phosphate-type chemical conversion film, such as a zinc phosphate-type chemical conversion film, or a manganese phosphate-type chemical conversion film. In the case of oil well steel pipes, it is preferred that a manganese phosphate-type chemical conversion film is formed on a joint portion of couplings and a zinc phosphate-type chemical conversion film is formed on a joint portion of oil well steel pipes.
According to another aspect, the present invention is a method of manufacturing a surface treated steel material wherein chemical conversion treatment is carried out on the surface of a steel material using a chemical conversion treatment liquid containing zinc and phosphoric acid or manganese and phosphoric acid, and further containing potassium.
The chemical conversion treatment liquid preferably has a molar concentration of potassium-containing ions of at least 6xc3x9710xe2x88x924% and at most 7xc3x9710xe2x88x921%.
The chemical conversion treatment may be carried out by immersing the steel material in the chemical conversion treatment liquid for at least five minutes at a temperature of 60-100xc2x0 C. and preferably of 70-100xc2x0 C.
Alternatively, the chemical conversion treatment may be carried out by supplying the chemical conversion treatment liquid to the steel material for at least five minutes at a temperature of 60-100xc2x0 C. and preferably 70-100xc2x0 C.
According to yet another aspect, the present invention is a chemical conversion treatment liquid for a steel material containing zinc and phosphoric acid or manganese and phosphoric acid and further containing potassium.
In a preferred embodiment, the molar concentration of potassium-containing ions in the chemical conversion treatment liquid is preferably at least 6xc3x9710xe2x88x924% and at most 7xc3x9710xe2x88x921%.
When the chemical conversion treatment liquid of this invention contains manganese and phosphoric acid and further contains potassium, the total acid number is preferably at least 30 and less than 55, and the ratio of the total acid number to the free acid number is preferably 3-15.